This invention relates to the sintering of powder metallurgy parts. The consolidation of metal powders into usable forms is a technique that has been used for thousands of years. Until metal working technology developed to the point where temperatures sufficient to enable melting and casting were able to be readily attained, consolidation of metal particles into a finished form was the only way possible to produce relatively high melting point materials. Even today, many ceramics and refractory metals are consolidated almost exclusively by powder techniques rather than by melting and casting. As a fundamental consolidation and forming process, P/M (powder metallurgy) enjoys certain unique characteristics which result in microstructural advantages compared to melting, casting and conventional working processes. These include elimination of macro and micro segregation, elimination of directionality of structure and finer resultant grain sizes. The end products of these advantages are better and more homogeneous mechanical and physical properties and better dimensional stability. In addition, because P/M fabrication usually uses close tolerance dies, the ratio of usable to scrap material is substantially higher than for other fabrication techniques. Besides being able to control the microstructure, the alloy chemistry can also be held to very close tolerances, even with alloying constituents which have widely varying melting points. One final advantage of P/M fabrication is that the process lends itself to the production of parts of a low cost and with surfaces that require very little, if any, final machining.
The production techniques involved in P/M will be discussed below. It is obvious, however, that one of the limiting factors in the production of P/M parts is the equipment necessary to do the consolidation, i.e., the initial pressing of the "green" compact and the further densification by sintering. The technique of preforming followed by conventional forging obviates the need for the sintering step. However, it requires a powder consolidation step to fabricate the preform blank.
Current powder fabrication techniques involving both metals and non-metals can be divided into two areas: those involving pressure and those that are pressureless. In addition, those techniques involving pressure can be done either hot or cold. Cold pressure forming includes vibratory compacting, cyclic compacting, die compacting with or without binders, powder rolling, isostatic pressing and explosive forming. Of these techniques, the most common is die compacting. The most serious limitation that all of these cold compaction techniques share is the difficulty in bonding hard or coarse particles. The interparticle bond is formed by pressing the particles together. These particles may not adhere to each other if, for instance, they have a surface layer as, for example, aluminum of if the particles are hard as with tungsten. In addition, die compaction has the disadvantage of high die cost, limited press capability, high unit costs for small production runs and limits on part sizes and shapes as a result of non-uniform pressing pressures due to powder-die friction. Lubricants are often used to overcome the friction problems but they degrade the final product and are expensive to remove. One further disadvantage of pressing to form porous bodies, is that it is most desirable to use spherical powders of a single size; consolidation of this type of powder is the most difficult using powder pressing techniques. Hot pressure forming of powders includes hot pressing, hot forging, hot isostatic pressing and powder extrusion. Besides sharing some of the disadvantages of cold pressure forming, hot pressure forming generally requires very complex and expensive equipment. The one technique which uses more or less conventional equipment is hot forging where P/M preforms are press forged to final shapes. The preforms can be produced either by cold pressure or pressureless forming.
Pressureless forming techniques have been employed to produce a body with sufficient green strength to be further sintered or used as preforms. Such techniques include loose powder sintering, slip casting and slurry casting. Porous metal parts have often been produced by loose powder sintering. The temperatures may be high and the times long, however, to obtain a part with sufficient strength for the application, be it a porous metal part or a preform. Slip and slurry casting are techniques used in the ceramic industry which have not been widely used in P/M due to their slowness and relatively high cost.
Sintering is defined as the heating of a particulate body below the melting point of at least one major constituent so as to cause interparticle bonding. Sintering can result in chemical, dimensional or phase changes as well as stress relief and alloying. Considerable theoretical and experimental work has been done on the mechanisms of sintering. For metals, the most important sintering mechanism has been determined to be vacancy diffusion which results in material transport by either bulk or surface diffusion. In considering a sinterable body, one must consider the components of the system, namely the solid particulate phase and the void or pore phase. The end product of such a solid-pore system will depend upon whether bulk or surface diffusion takes place. This will affect the morphology of the porosity. In bulk diffusion, vacancies can migrate either through the lattice or along grain boundaries. The net flow of vacancies from the pores of a sintering body to the surface results in a decrease in pore volume and an overall densification. On the other hand, if vacancy diffusion occurs as a surface diffusion phenomenon, the shape of pores will change to spherical to minimize surface energy but the pore volume will not decrease. Therefore, to obtain high density, bulk diffusion must control sintering while surface diffusion must be the important mechanism if a porous structure is desired. It should be noted that viscous flow will be an important mechanism if pressure is applied, as in hot pressing. In this case, densification also occurs.
Three stages of sintering have been defined. In the first stage, neck growth between particles occurs, no grain growth takes place and total shrinkage is but a few percent. In the intermediate stage, some grain growth occurs, the pore phase is continuous or open, and all pores are intersected by grain boundaries. At 85 to 95% density, the porosity changes from open to closed which begins the final stage of sintering. Studies on the initial stage of sintering have been made by others on a wide variety of both metals and non-metals including Cu, Ag, NaCl, Al.sub.2 O.sub.3, Fe.sub.2 O.sub.3, AgI, CaF.sub.2, NaF, Fe, Th and ice. In the metals, the initial sintering rate was found to be dependent upon the vacancy diffusion rate along grain boundaries. The greater the vacancy concentration and the finer the grain size, the more rapid the neck growth. In the intermediate sintering stage, pore shrinkage occurs resulting in an increased density as a function of sintering time and grain growth occurs with time as a function of t.sup.1/3. The densification rate is found to be initially linear with time but then decreases as the sintering time is increased. It has been assumed that the sintering mechanism is still bulk diffusion as in the initial sintering stage but the grain boundary component of the bulk diffusion rate becomes increasingly more important. Based on this, it seems reasonable to assume that grain growth inhibition would keep the sintering rate high by keeping the number of grain boundaries available for vacancy diffusion from decreasing. Analysis of the final stage of sintering has been complicated by several effects. One of these is discontinuous grain growth which can be responsible for the cessation of shrinkage before all pores have been removed from the material. Discontinuous grain growth is a result of one large grain growing at the expense of its smaller neighbors. The boundaries sweep across pores enclosing them within the grains. Another complicating factor in final densification is trapped gas. If the closed pores contain ambient gas trapped as a result of pore closure, the densification rate will decrease with increasing density. The bulk diffusion rate of the trapped gas will limit the rate of densification. It seems reasonable that grain boundaries intersecting the gas filled pores would act as diffusion paths for the trapped gas and allow it to diffuse to the surface more rapidly than if only lattice diffusion is available.
In summarizing the mechanisms involved in sintering in many of the common metals of technological importance, lattice diffusion of vacancies with the grain boundaries acting as sinks appears to be the most important aspect of the sintering process which explains pore and sample shrinkage for these materials. Any process which hopes to improve the sinterability and densification of these materials must affect the vacancy concentration and/or the number of grain boundaries. Improved sinterability, for example, has been observed in metal powders which have been cold worked. This would increase the vacancy concentration and reduce the grain size, thereby improving the rate of sintering in the first stage but, as grain growth occurs, the advantage of fine grain size would be lost. Surface oxidation of powders has been found to improve the sinterability of some metal powders sintered in a reducing environment. This would appear perhaps to be a vacancy generation mechanism. Techniques involving the addition of other constituents to improve the sinterability of powders have been investigated. This technique is generally referred to as "activated sintering".
The process variables of technological importance involved in sintering are temperature, particle size, atmoshpere and impurities or additives. Temperature is the most easily controlled variable. Without a knowledge of the kinetics of the different processes occurring in sintering, it is difficult to predict the effects of changing temperatures on the sintering behavior. For example, if the grain growth rate is affected differently than the vacancy diffusion rate for a given temperature change, the microstructures produced at different temperatures will be affected by which mechanisms are controlling. As stated above, a fine grain size and slow grain growth rate are desirable if densification is important. If, on the other hand, spheroidization of porosity is desired with no densification, rapid grain growth and slow vacancy diffusion would be a more desirable situation. Both the overall particle size and particle size distributions affect the sintering characteristics of a given material, all other sintering parameters being held constant. Generally speaking, the finer the powder size, the more rapid the sintering. The driving force for the sintering process is the lowering of the surface energy, which accounts for this size effect. While particle size distribution effects have not been explicitly analyzed, it is well known that a spread in particle size distribution leads to better packing and increased sintered densities.
The term activated sintering has been widely used without specifically defining the term. Two general categories of activated sintering have been defined: gaseous and solid. An example of gaseous activated sintering is the intentional addition of a halogen or halide to Fe powder. This lowers the sintering temperature. Ni bonded WC is an example of solid activated sintering. In conventional activated sintering, the intentionally added constituent increases the sintering rate by liquid or vapor phase transport. In addition, homogenization of multi-phase systems can occur. These mechanisms do not involve vacancy diffusion, hence are different than those encountered in conventional sintering of metals. However, the term activated sintering could also be applied to techniques which increase the surface area or tension, thereby increasing the driving force for conventional sintering or techniques which increase vacancy diffusion. Thus, the improvement in sinterability noted above the due to surface oxidation of the particles could be termed an activated sintering mechanism. One mechanism explaining improved sinterability after oxidation has been proposed. In this explanation, it is suggested that the surfaces of metal powders have an amorphous oxide layer which inhibits bonding and which is very difficult to reduce. By oxidizing at higher temperatures, a discontinuous oxide layer is formed which is easily reduced, thereby leaving a clean surface for activated sintering.